The present invention relates to a method and equipment for fabricating diffraction gratings, and more particularly to a method and equipment for fabricating diffraction gratings by two-beam interference, that is adapted to produce a DFB (distributed feedback) laser which consists of a homogeneous diffraction grating for the light source of optical fiber communications.
The conventional DFB laser shown in FIG. 1 operates at two different wavelengths at the same time, because the threshold currents for each wavelength .lambda. (or propagation constant .beta.) will degenerate with each other, as has been disclosed in H. Kogelnik et al., J. Appl. Phys., Vol. 43, pp. 2327-2335, 1972. To make a single longitudinal mode, therefore, it is an essential requirement to adjust the relative positions between the cleavage plane and the diffraction grating as has been experimentally demonstrated in T. Matsuoka et al., Jpn. Appl. Phys., Vol. 23, pp. L138-L140, 1984. To realize the single longitudinal mode, the positional precision of about.+-.100 angstroms is necessary, and it is difficult to adjust the relative positions relying upon the position of ordinarily employed cleavage. According to Haus et al.,IEEE J. Quantum Electronics, Vol. QE-12, pp. 532-539, 1976, on the other hand, it has been reported that the operation at the single longitudinal mode can be stably obtained if the diffraction grating is shifted in phase by .lambda./4 at nearly the center of the element. The diffraction grating having such a phase shift can be fabricated by a method of direct drawing based upon the electron beam disclosed in K. Sekartedjo et al., Electron Letter, 1983, or by a method which employs both a positive resist and a negative resist. With the method of direct drawing based upon the electron beam, however, the throughput is substantially small. With the method which employs a positive and negative resists, on a other hand, thickness of the resists becomes uneven, and the obtained diffraction gratings exhibit asymmetrical diffraction efficiency depending upon the right and left halves of the element, making it difficult to reproduce the single-mode operation.
An interference method can also be used as shown in FIG. 3. In this case, the laser beam emitted from a laser beam source 11 such as He-Cd laser or argon laser is split through a beam splitter 12, the beams after being split, are expanded for their diameter through beam expanders 131, 132, reflected by mirrors 141, 142 to change their directions of propagation, and are projected onto the surface of a specimen 15 to obtain fringed interference pattern. According to this conventional method without having a mechanism for adjusting the phase for the two light beams however, it is not possible to deliberately obtain diffraction gratings having dissimilar phases while maintaining the same period.